This invention relates to a rotational speed sensor and particularly to one for use on a motor vehicle for detecting the rotational speed of a ground engaging wheel. The invention further relates to such sensors having temperature sensing capabilities.
In numerous environments, there is a need to measure relative rotation between elements of a machine. For example, rotation sensors are used for jet engine main shafts, machine tool spindles, etc. Another application for such sensors is for anti-lock braking systems (ABS) which are increasingly popular on present day motor vehicles. Such systems are provided to automatically prevent wheel lock-up during hard braking maneuvers so that vehicle stability and directional control can be maintained. A critical feature of an ABS is a wheel speed sensor which provides an output to an ABS controller related to wheel rotation. Many passenger cars have such a sensor for each of their four wheels. Through such inputs, the braking system controller can determine if a wheel lock-up condition has occurred or is being approached and thus control the braking system. Wheel speed sensors are also used to provide inputs for traction control systems which reduce wheel spin during acceleration.
Numerous designs of ABS wheel speed sensors are presently known. Such sensors generally consist of a rotating part (rotor) in close proximity to a stationary part (stator). The rotating part, or "tone ring", has features which can be sensed as they pass the stationary part. Such features are typically ferromagnetic teeth, as on a gear, or magnetic poles which have been applied to the part. The stationary part includes a transducer which can detect the passing of the features of the tone ring as the tone ring rotates. The detection is indicated by an electrical signal which is emitted by the transducer. The transducer may be a variable reluctance device, Hall effect device, magneto-restrictive device, or of some other construction. Generally, the transducer is a device which senses magnetic fields or changing magnetic fields. Variable reluctance transducers are referred to as "passive" sensors in that they generate a voltage without being energized by an external source. "Active" sensors such as a Hall effect device are energized by an externally applied voltage and provide an output responsive to the magnetic fields passing through them.
Although sensors utilizing the above described technologies have been implemented with success, designers of such systems are constantly striving to increase their reliability, increase output signal strength, reduce packaging space requirements, facilitate production, assembly and calibration, all the while seeking to reduce their cost. Of principal concern is protecting the sensor elements from contaminants and environmental exposure. Road debris such as dirt, dust, salt and water can interfere with an unprotected sensor. In addition, wear debris from braking surfaces and significant temperature extremes are encountered by the sensor system. A present trend in wheel speed sensor design is to integrate the sensor into a wheel bearing assembly, enabling the system to be aligned and tested prior to being shipped to the vehicle manufacturer. Moreover, such an integrated configuration simplifies vehicle assembly on the assembly line and should increase reliability.
In modern day motor vehicle design, efficiency of packaging, light weight, ease of assembly, and reliability are of paramount importance. The hub and bearing assembly of this invention provides a compact and efficient construction and integrates many of the components of the bearing into the wheel speed sensor to provide an efficiently packaged unit. Moreover, the construction of the hub and bearing of this invention provides a sealed environment for the speed sensor, isolating it from environmental factors. The efficient magnetic configuration of the assembly of this invention provides a high level electrical output even at relatively low wheel speeds as compared with other variable reluctance type transducers.
Temperatures within vehicle wheel hubs can reach extreme levels as a result of bearing failures caused by lack of lubricant or other causes of high friction. Excessive temperatures can also be generated by the braking system caused, for example, by heavy use of the vehicle brakes in panic braking situations, or constant application of braking pressure on long down grades, especially in heavy-duty vehicles. If an indication of excessive wheel hub temperature is available, a driver could take corrective action. Although temperature sensing systems are presently known, they generally require the use of a separate sensing element such as a thermocouple or thermistor.
Another facet of the present invention is to utilize the coil winding of a wheel speed sensor transducer as a temperature sensitive element. The electrical resistance of the coil varies as a function of its temperature. By monitoring the coil resistance, the temperature may be determined. Several electrical circuit configurations are described to enable such resistance measurements.
In addition to motor vehicle applications, the features of this invention are believed applicable to a broad range of applications where a measurement of relative rotation is desired and/or where temperature measurements are needed.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.